Thermographic copying machines

ABSTRACT

A thermographic copying machine in which an electrically energized heating source is contained in a hollow radiation transparent roller engaged by a counter-pressure roller to provide a nip through which assemblies of sheets are passed, each such assembly including an original and a heat sensitive sheet, and a control circuit for supplying current to the electrically energized heating source. The control circuit includes a temperature sensing element adjacent to the surface of the hollow roll and in the path of radiant heat from said source and providing an output for reducing current to said source at least partly to counteract the increasing temperature to which sheet assemblies are subjected in passing between the rolls when the machine is in use over an extended period.

nit ed States Patent Crane et al.

541 THERMOGRAPHlC COPYING MACHINES [72] Inventors: Denis Percy Crane, Solihull; Edgar Joyce, Sutton Coldfield, both of England [73] Assignee: Oil-ex Group Limited, London, En-

gland 22 Filed: Feb. 16,1970

211 AppllNo.: 11,768

[30] Foreign Application Priority Data Feb. 22, 1969 Great Britain ..9,644/69 [52] US. Cl. ..219/216, 95/89 R, 219/471 [51] Int. Cl. ..H05b 1/00 [58] Field of Search..355/79; 95/89 R; 219/216, 388, 219/469, 470, 471

[56] References Cited UNITED STATES PATENTS 3,219,794 11/1965 Mindell et a1 ..219/216 3,475,589 10/1969 7 Bartusek et a1. ..95/89 X [451 Aug. 29, 1972 3,442,589 5/1969 lshikawa et al. .....95/89 X 3,465,122 9/1969' Kolless ..2l9/388X 3,417,226 12/1968 Thomiszer ..219/216 Primary Examiner.Samuel S. Matthews Assistant Examiner-Fred L. Braun Att0rneyMilton J. Wayne [57] ABSTRACT A thermographic copying machine in whichan electrically energized heating source is contained in a-hollow radiation transparent roller engaged by a counterpressure roller to provide a nip through which assemblies of sheets are passed, each such assembly including an original and a heat sensitive sheet, and a control circuit for supplying current to the electrically energized heating source. The control circuit includes a temperature sensing element adjacent to the surface of the hollow roll and in the path of radiant heat from said source and providing an output for reducing current to said sourceat least partly to counteract the increasing temperature to which sheet assemblies are subjected in passing between the rolls when the machine is in use over an extended period.

3 Claims, 4 Drawing Figures PATENTEMuszs m2 sum 3 or 3 NQU Q bm THERMOGRAPI'IIC COPYING MACHINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to therrnographic copying machines (hereinafter referred to as being of the kind specified) primarily for producing facsimilecopies required heat to the assembly of sheets at the heating station, and feed means for advancing the assembly of sheets along the feed path.

Various types of sheet assembly may be used in a machine of the kind specified depending upon the character of the facsimile copy to be produced. Thus,

for example, one assembly may be a simple copy sheet assembly for effecting reproduction on a copy sheet of material appearing on an original sheet. Another type may be a master sheet assembly for producing master sheets for use in hectographic duplicating machines in which case the material appearing on the master sheet is in reversed or mirror image relation to the matter appearing on the original sheet. Yet another type of assembly is one intended for the production of printing or duplicating stencil sheets, and a still further sheet assembly is one intended for the production of transparancies on which matter from an original sheet appears in a form suitable for display by projection using an optical projector.

The development of transfer of material presenting the facsimile copy on either the copy sheet, master sheet, stencil ortransparency is best effected to obtain clear copies of high definition at a predetermined temperature or within a predetermined range of temperatures at the heating station. This temperature, or this range of temperatures, may differ for the various classes of sheet assembly utilized, but in practice the problem arises in maintaining the required temperature (or a temperature within the required range) at the heating zone over a period of time in which successive I sheet assemblies are passed through the machine.

If a number of sheet assemblies are passed through in rapid succession the general temperature environment at the heating station tends to be raised, and it is thus common to find that the product, that is to say the copy sheet, master sheet, stencil, or transparency, has deteriorated in respect of the definition or delineation of the material produced thereon particularly where material is of a delicate character consisting of thin lines or other markings of a similar character.

2. Description of the Prior Art Attempts have been made to overcome this problem by varying the rate of feed of the sheet assemblies through the heating station, the rate of feed being increased automatically during the later stages of the senes.

There are, however, complications in such an arrangement, especially when it is required that various feed rates of predetermined values or within predeten mined ranges be pre-set for the purpose of ensuring proper reproduction for each of the various types of sheet assembly already mentioned.

The object of the present invention is to provide a new or improved therrnographic machine of the kind specified whereby the problem is overcome in a different manner having certain advantages as will hereinafter be apparent or pointed out in the following description.

SUMMARY OF THE INVENTION According to the invention we provide a thermographic machine of the kind specified characterized by the provision of temperature sensing means mounted in the machine ata position to be subjected to a temperature which varies with that to which easy assembly-of sheets is raised in passing through the heating station and providing a similarly varying output signal, and

control means responsive to said signal for reducing or raising the heat furnished by said source at least partly to counteract said temperature variation.

The invention is applicable primarily, but not exclusively, to a therrnographic machine of the kind specified wherein the heating station is defined by a pair of rolls, one of which is hollow and contains an electrically energized source of heat for example, an infra-red lamp, and is made of a material (for example glass) which will transmit the heat through the wall thickness of the roll, the other roll being a counterpressure roll providing in combination with the first roll a nip through which the assembly of sheets is passed. Asapplied to a therrnographic machine of the kind specified of this form, the control means comprises a circuit which includes a current controlling means settable in response to the output signal provided by the temperature sensing means.

The gradual increase in the temperature environment at the heating station arises from the fact that the roll which contains the heating source unavoidably absorbs a certain quantity of heat (as well as transmitting heat through the wall thickness of the roll). Heat is thus transmitted from the exterior surface of this roll by conduction to each assembly as it passes through the heating station and therefore the basic or datum temperature of the heat sensitive material applied to or incorporated in one or more of the sheets in the assembly 1 rises. Accordingly, when this material is subjected to an additional heat pattern in accordance with the matter appearing on the original sheet, the assembly leaves the heating station with the heat sensitive material at a still higher temperature which may be above the optimum temperature for ensuring high definition reproduction of the original sheet onto the copy sheet, master sheet, stencil or transparency, as the case may be.

A particularly simple and effective arrangement according to the invention for at least partly counteracting this effect, is one in which the temperature sensing means is so mounted as to be influenced by the surface temperature of the roll within which the heating source is mounted and also to receive radiant heat'from the heating source. In this way the sensing means is subjected to a temperature environment which at least approximates to and preferably closely corresponds to that of each sheet assembly at the heating station.

A further feature of the invention is that the current controlling means is preferably responsive to operation supply of current to the heating source over a period of time co-ordinated with the passage of the assembly of sheets through the heating station.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings, wherein: j

FIG. 1 shows in side elevation and partly in vertical cross-section one embodiment of thermographic machine of the kind specified in accordance with the invention,

FIG. 2 is a fragmentary diagrammatic view in plan showing the mechanism for efiecting adjustment of the speed of advancement to any oneof a number of preset values or ranges,

FIG. 3 is a circuit diagram of a circuit for supplying electrical current to the electrically energized heating means of the machine, and

FIG. 4 is a graphical representation of certain of the wave forms presented in the circuit and illustrated operation of the machine. I

As seen in FIG. 1 the machine comprises a body or supporting structure formed with approach and exit guide passageways l1 and 12 defining'a feed path for copy sheets through a heating station 13.

At the heating station are provided a pair of rolls l4 and 15, the latter being mounted in journals carried by lever arms 16 spring-loaded in an upward direction.

The roll may be formed from a resilient material such as heat-resisting rubber and the roll M is made of a material transparent to infra-red radiation, for example glass having a low co-efficient of thermal expansion, e.g., boro-silicate glass.

The roll 14! is driven from an electric motor 17 through the intermediary of a steplessly variable ratio gear 18, shaft 19 and worm and worm wheel 20, 21.

The worm'wheel 21 is fixed on a shaft on which is mounted a driving roll 22. The roll 22 together with a further roll 23 and a roll 15 serve to support and maintain the glass roll M in its proper position whilst it is driven rotatably about its horizontal axis.

Within the roll 14 is disposed an electrically energized heating source such as an infra-red lamp 24 around which is disposed a reflector 25, the reflecting surface 26 of which causes infrared radiation to be projected through an axially extending slot or mouth 27 afforded by the reflector. Such slot or mouth may be of uniform width along the major part of its axial length and may widen somewhat at each end.

A temperature sensing means is provided on the machine in such position or relation to the heating station that it undergoes temperature changes related to those which occur in each laterally extending zone of a sheet assembly when such zone is positioned at the heating station 27.

One factor which determines the temperature to which the laterally extending zone of each such assembly is raised, is the surface temperature of the roll 15 since this determines the basic or datum level of temperature over the entire surface of the particular sheet of each assembly which incorporates or is coated 4 with the heat sensitive material. The heat pattern, the configuration of which is determined by theIetter-press or other matter appearing on the original sheet, is determined primarily by the radiant heat transmitted through the wall of the roll 15. In the arrangement now illustrated the temperature sensing means senses the surface temperature of the roll 14 and comprises a thermistor Th. supported by a bracket 28 and spaced from the roll by a small radial distance typically onesixteenth of an inch (1.6 mm) at a position approximately mid-way between its ends and conveniently but not essentially between the two rolls 22 and 23. The sensing means also receives radiant heat from thelamp 24 through a hole 25a in the reflector, such radiation passing through the wall of the roll 14.

The temperature environment in which the sensing means is placed thus corresponds closely to that existing for each laterally extending zone of each sheet assembly as it passes through the heating station.

An output signal developed by the thermistor and dependent upon the temperature is used to control a supply of current to the lamp 24 by a control circuit illustrated in FIG. 3 as hereinafter described, the ar rangement being such that the supply of current to the lamp is decreased as the surface temperature of the roll 14 rises.

The motor 17 is mounted on a guide bar 29 for movement in the direction of the arrow 30 in order to bring different axial zones of the friction cone 31 into contact with rubber-tired driving wheel 31a of the steplessly variable gear 18. For the purpose of effecting positional adjustment of the motor 17 and hence variation in the speed at which the roll 14 is driven, a manually operable control member 32 is provided at the front of the machine, slidable along a slot in the front wall 33 of the body or supporting structure. The control member 32 is connected by a bell crank lever 34 and a link 35 to an upwardly projecting shift bar36 on the motor 17 or a mounting bracket therefore.

The front'wall 33 of the body or supporting structure may carry a. scale or indicator panels 36a, 36b, 36c, 36d, each indicating a particular position of the control member 32, or a range of positions, along the path of lateral adjustment corresponding to the upper and lower limits for a particular type of sheet assembly. For example, panel 360 representing the highest speed or speed range advancement of the sheet assemblies may be applicable to the reproduction of hectographic master sheets, panel 36b applicable to sheet assemblies for reproduction of original matter directly onto copy sheets and 36c applicable to sheet assemblies for producing duplicating or printing stencils. It is contemplated that the machine may be used for other purposes, for example sheetassemblies for laminating that v is causing two or more sheets to adhere to each other by the use of a heat-sensitive adhesive incorporated in or applied to the sheets as necessary.

Referring now to the circuit shown in FIG. 3, the arrangement of this will best be understood by a description of the functioning thereof, as follows.

Power is supplied to the circuit from any suitable supply such as a supply mains at terminals T1 furnishing alternating current at $0 to 60 Hz, and typically at volts or 240 volts. The supply current is subjected to full wave rectification in a diode bridge W1 to W4 and is applied between the cathode and anode of a thyristor SCRl through the filament R1 of the infra-red lamp 24 and noise supressing inductance L1, L2. The trigger electrode of the thyristor is connected to the movable contact of relay contacts A1, and when in the position shown, the thyristor presents an open circuit (high impedance to the load R1) and no current is supplied to the latter. A

Passage of a sheet assembly along the guide passageway 11 brings the leading edge of the sheet assembly into contact with a feeler member 37 compris ing an upstanding finger fixed to a horizontal spindle 38 for operating a normally open microswitch S1. When this switch is closed the energizing winding of relay All is energized through diode D2 and simultaneously capacitor C2 is charged through resistor R1 1 to a voltage determined by the resistor chain R2, R8, R9 and which is typically 24 volts stabilized by zener diode ZDl.

Energization of relay All causes the movable contact of contacts Al to move to the upper fixed contact.

The thermistor Th is connected in the base circuit of emitter-follower transistor T1 through variable resistor R10. The resistive value presented by Th and resistor R10 determines the emitter current and hence the voltage appearing at the junction of resistors R6 and R7 and thecharging current 1, flowing to capacitor C1 through diode D1. A further component of charging current 1 flows to capacitor Cl through resistor R5.

When the voltage appearing across capacitor C1 rises tov a predetermined value the control electrode of trigger generator UJTl (a unijunction semi-conductor) causes UJTl to present a virtual short circuit between bases B1, B2, and accordingly a positive pulse of voltage appears at the upper end of R4 (base B2) and is applied to the trigger electrode of SCRl.

The particular instant at which this pulse is generated in each half period depends upon the magnitude of the variable component 1 of charging current flowing through diode D1 to capacitor C1. As illustrated in FlG. 4, where two successive half periods of rectified supply voltage are shown in section A, section B illustrates the generation, of a triggering pulse at an early stage in each half period. This will occur when the surface temperature of the roll 14 is relatively low and hence when the thermistor Th has a low resistive value. The early incidence of the trigger pulse establishes conduction through the load R1 for a high proportion of each period.

in section C of FIG. 4 each trigger pulse is illustrated as being generated later in the respective half period, due to the incidence of a higher surface temperature at the roll 14 and possibly to higher temperature of the lamp 24 since this will be operating in a higher temperature environment. Each of these effects produces a higher resistive value for the thermistor Th.

It will thus be evident that the position of the trigger pulse controls the mark/space ratio of the pulses of current generated in the pulse forming sub-circuit of thyristor SCRl and hence the average current flowing through the load R1.

Upon release of the feeler member 37 when the trailing edge of a sheet assembly has passed beyond this, the

switch S1 is opened but the relay All does not immediately de-energize. it will, however, do so after a delay period determined by the time constant of the combination R11, C2. This is selected to ensure the movable contact of the contacts A1, remains in the upper position long enough to establish proper heating of the trailing margin of the sheet assembly, that is to say the portion of the'sheet assembly which extends between the heating station 27 and the feeler member 37 when the latter is just released.

It will be evident that the control circuit admits a very flexible adaption to the requirements of a plurality of diflerent types of sheet assembly. The resistor R10 is preferably a variable resistor which can be pre-set to enable the magnitude of compensating effect produced by the thermistor Th (in reducing the conduction angle of SCRl, as the surface temperature of the roll 14 rises), to be varied as will best suit the particular type of sheet assembly in use in the machine.

The functioning of the control circuit is independent of the adjustment means determining the speed of advancement of the sheet assemblies along the feed path and consequently a simple pre-set mechanism as illus trated in FIG. 2 can be utilized for the latter without complicating and increasing the cost of the machine.

It will be understood that whilst the siting of the temperature sensing means at the position shown in FIG. 1 is convenient in that it is accessible and does not interfere with the operation of moving parts of the machine and also enables it to receive the proper relative amounts of heat from the surface of roll 14 (partly by radiation and partly by convection) to and from the lamp 27 (by radiation) to undergo temperature variation closely corresponding to that of the lateral zone of each sheet assembly at the heating station other arrangements could be adopted. 4

For example, the heat sensing means could be sited inside roll 14 to receive heat from the inner surface and also from the lamp (with suitable shielding if necessary). Alternatively, it could be sited near the heating sta tion at a position above or below the exit section 12 at the feed path possibly to contact the sheet assemblies.

Further, instead of controlling the supply or current to the lamp a movable shield could be interposed between this and the heating station. The shield may be movable either directly by the sensing means when the latter is of a form to provide a mechanical output e.g., a bimetal strip or other thermo-motive member or indirectly by a motor controlled by the output signal of the sensing means.

We claim:

1. A thermographic machine comprising a body including means for defining a feed path through a heating station in the machine, feed means for advancing heat sensitive sheet assemblies therealong, said heating station being defined by a pair of rolls, at least one of said rolls being hollow and having a wall for engaging said sheet assemblies, and electrically energized heating means within said one roll for furnishing radiant heat at the heating station, the improvement comprismg:

a. a temperature sensing means, I

b. means for mounting said temperature sensing means at a position adjacent to said wall and also in the path of radiation from said heating means to provide an output signal which is dependent upon a combination of the temperature of said wall and radiant heat from said heating means,

0. control means responsive to said output signal for reducing and raising the heat .furnished by said heating means in response respectively to the rise and fall of the temperature sensed by said sensing means at least partly to counteract the temperature variation of said sheet assemblies at said heating station,

d. said means comprising an electrically energized heating element,

e. said hollow roll being made of a material which will transmit radiation from said heating element to each of said sheet assemblies,

f. said control means comprising a circuit in which g. said current controlling means comprising a pulse forming subcircuit for supplying pulses of current i. said circuit including a switch means,

j.said switch means being operable to cut off and establish respectively a supply of current to said electrically energized heating means,

k. said switch means has an operating member disposed in an approach section of said feed path to said heating station, so as to be operable by each sheet assembly when advanced therealong by said feed means,

1. said circuit further includes a time delay subcircuit,

111. said time delay sub-circuit is electrically connected with said switch means and with said current controlling means to maintain establishment of current through said heating means for a predetermined period after reversion of said switch means to an unoperated condition, upon passage of a trailing edge of said sheet assemblies past said operating member whereby said electrically energized heating means is maintained energized for a period sufficient to ensure proper reproduction of matter contained in the trailing margin of an original sheet included in each of said sheet assemblies.

3. A therrnographic machine comprising a body including means for defining a feed path through a heating station in the machine, feed means for advancing heat sensitive sheet assemblies therealong, said heating station being defined by a pair of rolls, at least one of said rolls being hollow and having a wall for engaging said'sheet assemblies, and electrically energized heating means within said one roll for furnishing radiant heat at the heating station, the improvement comprismg:

a. a temperature sensing means,

to said heating means, 20

h. said circuit including means for varying the mark/space ratio presented by-said pulses of current in dependence upon the magnitude of said output signal from said temperature sensing means.

2. A thermographic machine comprising a body including means for defining a feed path through a heating station in the machine, feed means for advancing heat sensitive sheet assemblies therealong, said heating station being defined by a pair of rolls, at least one of said rolls being hollow and having a wall for engaging said sheet assemblies, and electrically energized heating means within said one roll for furnishing radiant heat at the heating station, the improvement comprismg:

a. a temperature sensing means,

b. means for mounting said temperature sensing means at a position adjacent to said wall and also in the path of radiation from'said heating means to provide an output signal which is dependent upon a combination of the temperature of said wall and radiant heat from said heating means, a

c. control means responsive to said output signal for reducing and raising the heat furnished by said heating means in responsive respectively to the rise and fall of the temperature sensed by said sensing means at least partly to counteract the temperature variation of said sheet assemblies at said heating station,

d. said heating means comprising an electrically energized heating element,

e. said hollow roll being made of a material which will transmit radiation from said heating element to each of said sheet assemblies,

b. means for mounting said temperature sensing means at a' position adjacent to said wall and also in the path of radiation from said heating means to provide an output signal which is dependent upon a combination of the temperature of said wall and radiant heat from said heating means,

c. control means responsive to saidoutput signal for reducing and raising the heat furnishedby said heating means in response respectively to the rise and fall of the temperature sensed by said sensing means at least partly to counteract the temperature variation of said sheet assemblies at said heating station,

d. said feed means for advancing said sheet assemblies including adjustment means for varying the speed of advancement,

e. said adjustment means being operable by a manually movable control member,

f. indicator means being provided in operative asi. said control means comprising a circuit in which sociation with Said manually operable CQHU'OI said element is connected and which includes a member indicating a plurality different current controlling means settable in response to peed settings or ranges thereof for different said output signal provided by said temperature clifsses sensing means, g. said feed means for advancing said sheet assemg. said current controlling means comprising a pulse blies comprising forming sub-circuit for supplying pulses of current a motor, to said heating means, ii. a rotatable drive element on and driven by said h. said circuit including means for varying the motor,

mark/space ratio presented by said pulses of current in dependence upon the magnitude of said output signal from said temperature sensing means,

9M 10 engagement with each other for transmitting a thereof into rolling engagement with said driven drlvmg therethmugh from the motor to element and thereby steplessly vary the velocity the driven element, at least one of said ti afford d between said elements,

penphera surfaces bemg of generally comcal vi. means connecting said manually movable conform trol member with said motor to move same to v. motor mounting means providing for movement t for diff r m classes of work of said motor and said driving element collecf g g z q i z tively in a direction axially of said generally m ca 6 y 1 r conical surface to bring different axial zones 

1. A thermographic machine comprising a body including means for defining a feed path through a heating station in the machine, feed means for advancing heat sensitive sheet assemblies therealong, said heating station being defined by a pair of rolls, at least one of said rolls being hollow and having a wall for engaging said sheet assemblies, and electrically energized heating means within said one roll for furnishing radiant heat at the heating station, the improvement comprising: a. a temperature sensing means, b. means for mounting said temperature sensing means at a position adjacent to said wall and also in the path of radiation from said heating means to provide an output signal which is dependent upon a combination of the temperature of said wall and radiant heat from said heating means, c. control means responsive to said output signal for reducing and raising the heat furnished by said heating means in response respectively to the rise and fall of the temperature sensed by said sensing means at least partly to counteract the temperature variation of said sheet assemblies at said heating station, d. said means comprising an electrically energized heating element, e. said hollow roll being made of a material which will transmit radiation from said heating element to each of said sheet assemblies, f. said control means comprising a circuit in which said element is connected and which includes a current controlling means settable in response to said output signal provided by said temperature sensing means, g. said current controlling means comprising a pulse forming sub-circuit for supplying pulses of current to said heating means, h. said circuit including means for varying the mark/space ratio presented by said pulses of current in dependence upon the magnitude of said output signal from said temperature sensing means.
 2. A thermographic machine comprising a body including means for defining a feed path through a heating station in the machine, feed means for advancing heat sensitive sheet assemblies therealong, said heating station being defined by a pair of rolls, at least one of said rolls being hollow and having a wall for engaging said sheet assemblies, and electrically energized heating means within said one roll for furnishing radiant heat at the heating station, the improvement comprising: a. a temperature sensing means, b. means for mounting said temperature sensing means at a position adjacent to said wall and also in the path of radiation from said heating means to provide an output signal which is dependent upon a combination of the temperature of said wall and radiant heat from said heating means, c. control means responsive to said output signal for reducing and raising the heat furnished by said heating means in responsive respectively to the rise and fall of the temperature sensed by said sensing means at least partly to counteract the temperature variation of said sheet assemblies at said heating station, d. said heating means comprising an electrically energized heating element, e. said hollow roll being made of a material which will transmit radiation from said heating element to each of said sheet assemblies, f. said control means comprising a circuit in which said element is connected and which inCludes a current controlling means settable in response to said output signal provided by said temperature sensing means, g. said current controlling means comprising a pulse forming sub-circuit for supplying pulses of current to said heating means, h. said circuit including means for varying the mark/space ratio presented by said pulses of current in dependence upon the magnitude of said output signal from said temperature sensing means, i. said circuit including a switch means, j. said switch means being operable to cut off and establish respectively a supply of current to said electrically energized heating means, k. said switch means has an operating member disposed in an approach section of said feed path to said heating station, so as to be operable by each sheet assembly when advanced therealong by said feed means, l. said circuit further includes a time delay subcircuit, m. said time delay sub-circuit is electrically connected with said switch means and with said current controlling means to maintain establishment of current through said heating means for a predetermined period after reversion of said switch means to an unoperated condition, upon passage of a trailing edge of said sheet assemblies past said operating member whereby said electrically energized heating means is maintained energized for a period sufficient to ensure proper reproduction of matter contained in the trailing margin of an original sheet included in each of said sheet assemblies.
 3. A thermographic machine comprising a body including means for defining a feed path through a heating station in the machine, feed means for advancing heat sensitive sheet assemblies therealong, said heating station being defined by a pair of rolls, at least one of said rolls being hollow and having a wall for engaging said sheet assemblies, and electrically energized heating means within said one roll for furnishing radiant heat at the heating station, the improvement comprising: a. a temperature sensing means, b. means for mounting said temperature sensing means at a position adjacent to said wall and also in the path of radiation from said heating means to provide an output signal which is dependent upon a combination of the temperature of said wall and radiant heat from said heating means, c. control means responsive to said output signal for reducing and raising the heat furnished by said heating means in response respectively to the rise and fall of the temperature sensed by said sensing means at least partly to counteract the temperature variation of said sheet assemblies at said heating station, d. said feed means for advancing said sheet assemblies including adjustment means for varying the speed of advancement, e. said adjustment means being operable by a manually movable control member, f. indicator means being provided in operative association with said manually operable control member for indicating a plurality of different speed settings or ranges thereof for different classes of work, g. said feed means for advancing said sheet assemblies comprising i. a motor, ii. a rotatable drive element on and driven by said motor, iii. a rotatable driven element drivingly connected with said at least one of said rolls, iv. the respective peripheral surfaces on said drive element and said driven element being in rolling engagement with each other for transmitting a driving motion therethrough from the motor to the driven element, at least one of said peripheral surfaces being of generally conical form, v. motor mounting means providing for movement of said motor and said driving element collectively in a direction axially of said generally conical surface to bring different axial zones thereof into rolling engagement with said driven element and thereby steplessly vary the velocity ratio afforded between said elements, vi. means connecting said manually movable control member with said motor to move same to diffErent positions for different classes of work indicated by said indicator means. 